Restart the computer then immediately hold the Command key and the letter R key together until you see the Apple Logo. This will boot to the Recovery HD partition. When it boots you will see a window called Utilities, select the Disk Utility and then select Continue. Then in Disk Utility select Macintosh HD in the left column and then select Verify Disk on the right. If that is OK in green type then select Repair Permissions. If all is ok then Verify Disk on the iomega and repair if necxessary.
Let me know.
Thanks for coming back so quickly. Because of time pressures this pm I will not be able to try this till tomorrow.
When you say "repair if necessary", what do you mean or will it be obvious what I should do?
I have tried your suggestion. I ran the Disk Utility through to "Permission Verification Complete". But neither in the recovery mode nor in normal operation is the iomega drive seen. (If it is relevant, while running Repair etc no "OK in green type" appeared.)
No, only the HD and Superdrive